1. Field of the Invention
The present invention relates to graphic display controllers and, more particularly, to integrated circuits that implement graphics display controllers and programmable clock signal generators.
2. State of the Art
In the field of computer graphics, the prevailing present-day standard is the VGA standard, which specifies a 640.times.480 pixel display format. To implement that standard, conventional practice has been to use a VGA video interface card to interface a computer to a high-resolution display device. A VGA interface card is typically built around a VGA "chip set" that includes a number of complementary chips proven to work well together and that together realize all of the necessary functions to drive the display device. Reduction of the number of chips in the chip set simplifies manufacture, reduces cost and increases reliability.
In some VGA interface cards of the prior art, the chip set has included a programmable clock signal generator chip, or "P clock". An example of such a P clock chip is the Dual Video/Memory Clock Generator ICS90C64 manufactured by Integrated Circuit Systems, Inc. of Valley Forge Pa. A P clock chip simultaneously generates two clock signals. One clock signal is for the video memory used to store display information, and the other clock signal is used as the video dot clock signal, or pixel clock. Each cycle of the pixel clock, signals are produced to display one pixel on the display screen.
other than P clock chips, chips are also available to generate the video dot clock only, i.e. V clock chips. An example of one such chip may be found in U.S. Pat. No. 5,036,216, incorporated herein by reference. Clock chips of the type referred to are essentially frequency generators and are designed such that the frequency generated may be selected from among a set of predetermined frequencies that includes frequencies suitable for most common applications. The clock chips are therefore programmable in the narrow sense that their operating frequency can be selected. (The word "programmable" as it appears herein is used in the foregoing sense unless other indicated.)
The primary chip in a VGA chip set is typically a large, sophisticated, digital VGA controller chip manufactured using VLSI techniques. By contrast, P clock chips are comparatively small and are at least partly analog in order to perform the function of frequency generation. Therefore, despite the pressure to reduce chip count, the P clock chip and the VGA controller chip have been unlikely candidates for integration.